The present invention is related to a railway vehicle brake system of the type combining electric and friction braking and particularly to a system in which the electric and friction braking is continuously blended by reason of the friction brake supplementing the electric brake an amount dependent upon the deficiency of the electric brake in satisfying the brake command.
Such brake blending arrangements generally fall into two classes, one in which the brake command and degree of available electric brake are compared electrically to obtain an error signal, in response to which the friction brake is applied, and the other in which the comparison is accomplished pneumatically. These classes of brake blending arrangements are depicted in FIGS. 1 and 2 of U.S. Pat. No. 4,073,541, issued Feb. 14, 1978. In the latter one of the above-mentioned classes, the pneumatic brake command signal for each car is derived from a continuous pipe that passes through each car of the train. It is well known that such trainline pipes experience a higher incidence of breakage than other car mounted piping, due in part to the hose coupling and associated fittings between the respective cars and the relative movement between the cars. Since a loss of the pneumatic brake command signal occurs with breakage of a trainline pipe, it follows that the friction brake pressure is also depleted due to the error signal, produced in accordance with the deficiency of the electric brake relative to the brake command, being accordingly reduced to zero. This of course is an undesirable characteristic, since without the availability of a friction brake, at least as a back-up brake, a train is in danger of being without any brakes whatsoever, particularly at low speeds when the effectiveness of the electric brake typically fades.